Concern over the availability and environmental consequences of fossil fuel energy sources has generated interest in recent years in the search for and development of alternative energy sources which would complement, and perhaps replace, fossil fuel sources. Numerous alternative energy sources have been contemplated including both nuclear and solar energy. Several approaches directed toward the use of solar energy have been and are being investigated. For example, solar energy devices may be used to produce electricity either directly through photovoltaic devices or indirectly through thermal devices. A third approach is photoelectrolysis and involves the use of sunlight to split water into oxygen and hydrogen. After the water is split, the hydrogen and oxygen may be collected and the hydrogen utilized as a fuel. This approach is attractive because hydrogen is more easily stored than electricity or heat and is easily used as a fuel.
Although many structures and material combinations have been contemplated, the devices presently considered useful for photoelectrolysis may be described briefly as having two electrodes immersed in an electrolyte and connected via an external electrical circuit. One of the electrodes will be formed from a semiconductor material and shining sunlight on it causes creation of electron-hole pairs as the incident light is absorbed. At one electrode, electrons move to the electrolyte and hydrogen is produced while at the other electrode, holes move to the electrolyte and oxygen is produced. This description assumes that the flatband potentials are above the H.sup.+ /H.sub.2 potential and no external bias is needed to produce H.sub.2 and O.sub.2. If the flatband potential is below the H.sup.+ /H.sub.2 potential, an external bias will be required.
Although Becquerel constructed the first photoelectrochemical cell in 1839, most work with such cells has been of a fundamental nature and did not lead to widespread commercial use of such cells. However, in recent years, several systems have been proposed that are more serious candidates for commercial use. For example, Fujishima and Honda, Nature, 238, pp. 37-38, July 7, 1972, proposed to use a cell having a TiO.sub.2 electrode and a platinum counterelectrode to produce oxygen at the TiO.sub.2 electrode and hydrogen at the platinum electrode. In 1976, several groups disclosed devices using a strontium titanate (SrTiO.sub.3) electrode in a cell similar to that of Fujishima and Honda to generate hydrogen and oxygen.
One of the recurring problems in producing useful photoelectrochemical cells is the possibility that one of the electrodes, generally composed of a semiconductor material, may undergo irreversible decomposition. Such decomposition did not occur for the TiO.sub.2 or SrTiO.sub.3 cells, but they suffered from another problem, namely, poor solar to hydrogen conversion efficiency. Both materials have large bandgaps, approximately 3 eV, and are responsive to only a relatively small percentage of the incident sunlight. Thus, stabilization of cells using semiconductor materials having smaller bandgaps would be desirable.
Another approach, which attempted to increase the conversion efficiency, is described by Bookbinder, et al in Journal of the American Chemical Society, pp. 7721-7723, Dec. 19, 1979. This approach uses a semiconductor having a bandgap lower than that of TiO.sub.2 and also involves the manipulation of the charge-transfer kinetics by either a sacrificial reagent or catalyst. The paper reported a cell having a p-type silicon photocathode and a platinum anode separated by a separator. The three components were immersed in an aqueous electrolyte with one electrode in each of the two electrode compartments. The photocathode compartment further comprised N,N'-dimethyl-4,4,4'-bipyridinium (PQ.sup.2+). PQ.sup.2+ is photoreduced at the p-type silicon electrode and the reduced PQ.sup.+ reacts with water over a catalyst to form gaseous hydrogen as the PQ.sup.+ is oxidized from PQ.sup.+ to PQ.sup.2+. An external applied voltage of at least approximately 0.8 volts is required to reach the 1.23 volts thermodynamically required to split H.sub.2 O.